Generally, lubricating oils are designed to reduce friction between moving automotive components and protect their surfaces by covering them with a film of lubricant. Generally, lubricating oils are also designed to prevent or reduce wear of moving surfaces by creating a chemical film that facilitates shearing at the interface, instead of shearing through the asperities of the contacting surfaces. The oil may also serve other functions such as preventing corrosion by neutralizing acids that are formed at hot spots, improving sealing at some interfaces, cleaning the rubbing surface and transporting the waste products out of the contract zone, and carrying heat away from hot surfaces. These vast requirements necessitate different compositions and physical properties for the lubricant for performing various required lubrication functions.
The concept of nanofluids, i.e., nanoparticle-fluid dispersions, was introduced in the mid 1950's at the Argonne National Laboratory. Compared with millimeter- or micrometer-sized particle suspensions, nanofluids generally possess improved long term stability, much higher surface area, as well as improved mechanical, thermal and rheological properties. However, recent research efforts on nanofluids have mainly been focused on the preparation and evaluation of water or ethylene glycol (EG)-based nanofluids while reports of the synthesis of oil-based nanofluids are relatively uncommon.
There have been several mechanisms contemplated in the literature by which dispersed nanoparticles in lubricants result in lower friction and wear. These mechanisms include: formation of a transferred solid lubricant film from nanoparticles under the contact pressure, rolling of spherical nanoparticles in the contact zone, reducing asperity contact by filling the valleys of contacting surfaces, and shearing of nanoparticles at the interface without the formation of an adhered film.
A new mechanism for the role of solid lubricant nanoparticles was recently proposed. According to the proposed mechanism, one role of solid lubricant nanoparticles in oils and greases is to break apart the wear agglomerate that is commonly formed at the sliding interface. The wear agglomerate, sometimes referred to as the transferred film, is normally adhered to the harder surface. The entrapment of the wear agglomerate reduces the contact area which in turn causes the normal contact pressure to be increased. Therefore, the plowing of the mating surface by the wear agglomerate is enhanced. The enhanced plowing increases friction and wear. The wear debris agglomeration process and some factors that affect it are discussed in the literature.
However, in addition to preventing wear and lubricating the surfaces, it is also often desirable to improve performance of the lubricated surfaces. In this regard, it may be desirable to minimize the overall lubricant film thickness to improve fuel economy and other performance factors. However, depending on the film thickness and the roughness of the surfaces, the surfaces may experience undesired wear.